Polymerization catalyst used for producing polyolefins that have excellent combined properties

ABSTRACT

The invention relates to a process for preparing a catalyst for the polymerization of olefins, which comprises:
         a) preparation of a finely divided silica xerogel,   b) loading of the xerogel with chromium from a solution of chromium trioxide or a chromium compound which is converted into chromium trioxide under the conditions of step c), and   c) activation of the resulting product at from 400 to 1100° C. in a water-free gas stream comprising oxygen in a concentration of above 10% by volume,
 
wherein a fluoride doping with a fluorinating agent is carried out in step b) or in step c). Furthermore, the invention relates to a catalyst for the polymerization of olefins which is obtainable by the process of the invention and to a process for the polymerization of olefins in which an olefin or an olefin mixture is polymerized in the presence of a catalyst according to the invention.

The invention relates to a polymerization catalyst based on chromiumcatalysts supported on silica gel.

Polymerization catalysts comprising silica gel or modified silica gel assupport material and chromium as active component play an important rolein the preparation of high density polyethylene (HDPE). The conditionsunder which the supports and the catalysts are prepared determine thechemical composition, pore structure, particle size and shape of thecatalysts. Prior to the polymerization, the catalysts are activated athigh temperatures to stabilize chromium as Cr(VI) species on thecatalyst surface. This species is reduced by addition of ethylene or ofreducing agents to form the catalytically active species which catalyzesthe polymerization. The composition of the catalyst, its structure andthe activation conditions have a critical influence on the performanceof the catalyst in the polymerization process, the activity of thecatalyst, the structure and the properties of the polymer formed.

Despite intensive studies, the exact details as to how these catalystsinfluence the polymerization kinetics and polymer properties have notyet been fully elucidated. Small changes in the composition or structureof the catalyst therefore frequently lead to surprising effects.

U.S. Pat. No. 3,130,188 relates to a chromium oxide catalyst supportedon silica gel, which is, before it is activated, doped with inorganicfluorides such as ammonium hexafluorosilicate. This catalyst displays anincreased polymerization activity and the polyethylene obtained has anarrow molecular weight distribution.

In Advances in Catalysis, 33, 47-98, 1985, M. P. McDaniel discloses thatfluorine-doped Cr(VI)/silica gel catalysts have an increased activity atlow calcination temperatures. Furthermore, it is stated that theelectronic environment of the chromium is changed significantly by thefluoride doping, which is given as an explanation of a suppressedtermination rate. The molecular weight distribution of some of thepolymers obtained becomes narrower, which indicates a more uniformenvironment around the chromium.

DE-A 25 40 279 relates to a process for preparing a catalyst for thepolymerization of olefins based on Cr(VI)/silica gel, in which thesupport material is a silica xerogel which is prepared in a specificway. This xerogel is loaded with chromium trioxide or a chromiumcompound which is converted into chromium trioxide on subsequentactivation and is subsequently activated at from 400 to 1000° C. in awater-free gas stream comprising oxygen in a concentration of >10% byvolume.

The invention starts out from a process for preparing a catalyst for thepolymerization of olefins as described in DE-A 25 40 279, whichcomprises:

-   a) preparation of a finely divided silica xerogel by    -   a1) use of a particulate silica hydrogel which contains from 10        to 25% by weight of solid (calculated as SiO₂), is largely        spherical and has a particle diameter of from 1 to 8 mm and is        obtained by        -   a11) introduction of a sodium or potassium water glass            solution into a rotating stream of an aqueous mineral acid,            both longitudinally and tangentially to the stream,        -   a12) spraying of droplets of the resulting silica hydrosol            into a gaseous medium,        -   a13) allowing the sprayed hydrosol to solidify in the            gaseous medium,        -   a14) freeing the resulting largely spherical particles of            the hydrogel of salts without prior aging by washing,    -   a2) extraction of at least 60% of the water present in the        hydrogel by means of an organic liquid,    -   a3) drying of the resulting gel up to until the dried product        experiences no further weight loss at 180° C and a reduced        pressure of 13 mbar no further weight loss occurs over a period        of for 30 minutes (xerogel formation)    -   a4) adjustment of the particle diameter of the xerogel obtained        to from 20 to 2000 μm,-   b) loading of the xerogel with chromium from a solution of chromium    trioxide or a chromium compound which is converted into chromium    trioxide under the conditions of step c), and-   c) activation of the resulting product at from 400 to 1100° C. in a    water-free gas stream comprising oxygen in a concentration of above    10% by volume.

It is an object of the present invention to further develop this processfor preparing a catalyst based on Cr(VI)/silica gel. The catalystobtained should be suitable for preparing polyethylene or ethylenecopolymers having an improved property profile. In particular,polyethylene having a balanced ratio of environmental stress crackingresistance to impact toughness should be obtained.

The process of the invention is characterized in that a fluoride dopingwith a fluorinating agent is carried out in step b) or c).

The process of the invention makes it possible to prepare polyolefins,in particular polyethylene or ethylene copolymers, having balancedproduct properties. In particular, polymers having a high environmentalstress cracking resistance combined with high impact toughness areobtained. These properties usually run counter to one another.

The preparation of the catalyst support and the application of thechromium are carried out as described in DE-A 25 40 279.

Step a)

In the first step of the preparation of the support material, it isimportant that a silica hydrogel which has a relatively high solidscontent of from 10 to 25% by weight (calculated as SiO₂), preferablyfrom 12 to 20% by weight, particularly preferably from 14 to 20% byweight, and is largely spherical is used. This silica hydrogel isprepared in a specific way which is described in steps a11) to a14). Thesteps a11) to a13) are described in more detail in DE-A 21 03 243. Stepa14), viz. washing of the hydrogel, can be carried out in any desiredway, for example according to the countercurrent principle using waterwhich is at a temperature of 80° C. and has been made slightly alkalineby means of ammonia (pH up to about 10).

The extraction of the water from the hydrogel (step a2)) is preferablycarried out by means of an organic liquid, which is particularlypreferably miscible with water, selected from the group consisting ofC₁-C₄-alcohols and C₃-C₅-ketones. Particularly preferred alcohols aretert-butanol, i-propanol, ethanol and methanol. Among the ketones,acetone is preferred. The organic liquid can also comprise a mixture ofthe abovementioned organic liquids, and in any case the organic liquidcontains less than 5% by weight, preferably less than 3% by weight, ofwater prior to the extraction. The extraction can be carried out incustomary extraction apparatuses, e.g. column extractors.

Drying (step a3)) is preferably carried out at from 30 to 140° C.,particularly preferably from 80 to 110° C., and at pressures ofpreferably from 1.3 mbar to atmospheric pressure. Here, because of thevapor pressure, an increasing temperature should be combined with anincreasing pressure and vice versa.

The setting of the particle diameter of the xerogel obtained (step a4))can be carried out by any appropriate method, e.g. by milling andsieving.

Step b)

The chromium trioxide is preferably applied to the xerogel from a0.05-5% strength by weight solution of chromium trioxide in aC₃-C₅-ketone or from a 0.05-15% strength by weight solution of achromium compound which is converted into chromium trioxide under theconditions of step c) in a C₁-C₄-alcohol, where the respective solventmust contain no more than 20% by weight of water. The xerogel issuspended in the solution of the respective chromium compound and theliquid constituents of the reaction mixture are evaporated while mixingcontinually and as homogeneously as possible. The residual moisturecontent, including the content of organic solvents, of the xerogel ladenwith the chromium component should be not more than 20% by weight ofvolatile constituents, preferably not more than 10% by weight ofvolatile constituents, based on the xerogel.

Suitable chromium components apart from chromium trioxide and a chromiumhydroxide are soluble salts of trivalent chromium with an organic orinorganic acid, e.g. acetates, oxalates, sulfates, nitrates. Particularpreference is given to those salts which on activation are convertedinto chromium(VI) without leaving a residue, e.g. chromium(III) nitratenonahydrate.

The catalyst obtained in step b) generally has a chromium content offrom 0.05 to 5% by weight, preferably from 0.1 to 1.5% by weight,particularly preferably from 0.2% by weight to 1% by weight, based onthe total mass of the catalyst.

Step c)

The activation of the catalyst can be carried out in a customary mannerunder conditions which should be such that the chromium in the finishedcatalyst is present essentially in the hexavalent state (Cr(VI)).

Activation is preferably carried out at from 400 to 1100° C., preferablyfrom 500 to 800° C., particularly preferably from 600 to 700° C., in awater-free gas stream comprising oxygen in a concentration of above 10%by volume, e.g. in air.

Fluoride Doping

The doping with fluoride can be carried out in step a), in step b) or instep c). In a preferred embodiment, doping is carried out in step b) byapplying a fluorinating agent together with the desired chromiumcomponent, for example by coimpregnation of the support with a solutionof the fluorinating agent and the desired chromium compound.

In a further preferred embodiment, the doping with fluorine is carriedout subsequent to the application of the chromium during activation instep c) of the process of the invention. Here, fluoride doping isparticularly preferably carried out together with the activation at from400 to 900° C. in air. A suitable apparatus for this purpose is, forexample, a fluidized-bed activator.

Suitable fluorinating agents are all customary fluorinating agents suchas ClF₃, BrF₃, BrF₅, (NH₄)₂SiF₆ (ammonium hexafluorosilicate), NH₄BF₄,(NH₄)₂AlF₆, NH₄HF₂, (NH₄)₃PF₆, (NH₄)₂TiF₆ and (NH₄)₂ZrF₆. Preference isgiven to using fluorinating agents selected from among (NH₄)₂SiF₆,NH₄BF₄, (NH₄)₂AlF₆, NH₄HF₂, (NH₄)₃ PF₆. Particular preference is givento using (NH₄)₂SiF₆.

The fluorinating agent is generally used in an amount of from 0.5 to 10%by weight, preferably from 0.5 to 8% by weight, particularly preferablyfrom 1 to 5% by weight, very particularly preferably from 1 to 3% byweight, based on the total mass of the catalyst used. The properties ofthe polymers prepared can be varied as a function of the amount offluoride in the catalyst.

Furthermore, the invention provides a catalyst for the polymerization ofolefins which is obtainable by the process of the invention and providesa process for preparing polyolefins in which the catalyst of theinvention is used. This catalyst is suitable for the homopolymerizationand copolymerization of olefins, preferably of ethene andC₃-C₈-α-monoolefins. The catalyst of the invention is particularlypreferably used in the polymerization of ethene. The polymerization canbe carried out in a customary manner, preferably in suspension or a dryphase. In general, the concomitant use of hydrogen as molecular weightregulator is not necessary, but such regulators can be used in smallamounts.

The use of the catalysts of the invention makes it possible to preparepolyolefins which have a balanced property profile. In particular, thecopolymerization of ethene with α-olefins using the catalyst of theinvention leads to polyethylene having a low flow rate of from 1 to 100g/10 min, preferably from 1.5 to 50 g/10 min, particularly preferablyfrom 2 to 30 g/10 min. Owing to the low flow rate, these polyethylenescan be processed very readily. The flow rate of the polyethylene formedcan be controlled via the fluoride content of the catalyst of theinvention. The greater the fluoride content in the catalyst, the lowerthe flow rate, i.e. the polymer properties can be controlled.

The other polymer properties are also influenced by the doping withfluoride. Thus, the polyethylene obtained using the catalyst of theinvention has, for example, a narrower molecular weight distributionthan the polyethylene prepared using the corresponding undopedcatalysts. Despite the narrow molecular weight distribution, the fatigueproperties of the polyolefins prepared using the catalyst of theinvention do not deteriorate. Products produced from these polyolefinshave a high creep strength. Furthermore, both the impact toughness andthe environmental stress cracking resistance are high despite the narrowmolecular weight distribution. This is surprising, since according to“Polymere Wirkstoffe”, editor H. Batzer, volume III, page 70, Table1.17, Thieme Verlag, Stuttgart, N.Y., 1984, the impact toughness usuallyincreases with a narrow molar mass distribution while the environmentalstress cracking resistance increases with a broader molar massdistribution.

The present invention further provides a process for the polymerizationof olefins, in which an olefin or an olefin mixture is polymerized inthe presence of the catalyst of the invention.

The polyethylene obtained by the process of the invention has a highenvironmental stress cracking resistance and a high impact toughness,which deserves particular emphasis since these properties usually runcounter to one another, i.e. when the environmental stress crackingresistance is increased, the impact toughness drops, and vice versa. Thepolyethylene obtained is suitable, for example, for producingblow-molded products such as canisters or other containers which comeinto contact with solvents or hazardous materials. Furthermore, thepolyethylene obtained is suitable for, inter alia, the production offilms, pipes, pipe linings and IBCs.

The catalysts prepared by the process of the invention thus make itpossible to prepare polyolefins, in particular polyethylene, havingoptimized product properties. The low flow rate and thus goodprocessability and a high environmental stress cracking resistancetogether with a high impact toughness deserve particular mention.

The following examples illustrate the invention.

EXAMPLES

1. Preparation of the Silica Xerogel

A mixing nozzle as shown in the figure in DE-A 2 103 243 and having thefollowing data is employed: the diameter of the cylindrical mixingchamber formed by a plastic hose is 14 mm, and the mixing chamber length(including after-mixing section) is 350 mm. A tangential inlet holehaving a diameter of 4 mm for the mineral acid is provided close to theinlet end of the mixing chamber, which has a closed end face. Next tothis are four further holes likewise having a diameter of 4 mm and thesame inflow direction for the water glass solution, with the distancebetween the holes being 30 mm, measured in the longitudinal direction ofthe mixing chamber. The ratio of length to diameter for the primarymixing zone is therefore about 10:1. For the adjoining secondary mixingzone, this; ratio is 15. As spray outlet piece, a piece of tube whichhas been pressed flat into a slight kidney shape is pushed over theoutlet end of the plastic hose.

325 l/h of 33% strength by weight sulfuric acid at 20° C. and under anoperating pressure of about 3 bar and also 1100 l/h of water glasssolution (prepared from technical-grade water glass containing 27% byweight of SiO₂ and 8% by weight of Na₂O by dilution with water) having adensity of 1.20 kg/l and a temperature of likewise 20° C. and under apressure of likewise about 3 bar are fed to this mixing apparatus.Neutralization in the mixing chamber lined with the plastic hose formsan unstable hydrosol having a pH of from 7 to 8 and this spends anotherapproximately 0.1 s in the after-mixing zone until completelyhomogenized before being sprayed into the atmosphere through the nozzleoutlet piece as a fanned-out liquid jet. During its flight through theair, the jet breaks up into individual droplets which, owing to thesurface tension, take on a largely spherical shape and solidify duringtheir flight within about 1 second to form hydrogel spheres. The sphereshave a smooth surface, are clear, contain about 17% by weight of SiO₂and have the following particle size distribution

>8 mm 10% by weight 6-8 mm 45% by weight 4-6 mm 34% by weight <4 mm 11%by weight(The particle size distribution can be varied at will by using othernozzle outlet pieces)

At the end of their flight, the hydrogel spheres are collected in awashing tower which is filled virtually completely with hydrogel spheresand in which the spheres are immediately, without aging, washed free ofsalts with slightly ammoniacal water at a temperature of about 50° C. ina continuous countercurrent process.

The spheres having a diameter in the range from 2 to 6 mm are isolatedby sieving and 112 kg of these spheres are placed in an extraction drumwith an inlet at the top, a screen bottom and a swan neck-shapedoverflow which is connected to the underside of the drum and keeps theliquid level in the drum sufficiently high for the hydrogel spheres tobe completely covered with liquid. Ethanol is then fed in at a rate of60 l/h until the density of the ethanol/water mixture flowing out fromthe overflow has dropped to 0.826 g/cm³; about 95% of the water presentin the hydrogel have then been extracted.

The spheres obtained in this way are then dried (12 hours at 120° C.under a reduced pressure of 20 mbar) until no weight loss occurs over aperiod of 30 minutes at 180° C. under a reduced pressure of 13 mbar.

The dried spheres are subsequently milled and the xerogel particleshaving diameters from 40 to 300 μm are isolated by sieving.

Application of the Active Component

The xerogel particles are treated with a 3.56% strength by weightchromium nitrate solution (Cr(NO₃)₃·9H₂O) in methanol for 5 hours andfreed of methanol under reduced pressure, so that the catalyst precursorobtained has a chromium content of 1% by weight of Cr, based on thetotal mass.

Activation and Doping

Activation is carried out with air at 600 or 650° C. in a fluidized-bedactivator. Fluoride doping using 1% by weight or 2% by weight ammoniumhexafluorosilicate (ASF) (fluoride content, based on the total mass ofthe catalyst) is carried out during activation. For the activation, thecatalyst precursor is heated to 350° C. over a period of 1 hour, held atthis temperature for 1 hour, subsequently heated to the desiredactivation temperature, held at this temperature for 2 hours andsubsequentially cooled, with cooling below 350° C. being carried outunder N₂.

Table 1 summarizes the catalysts prepared (activation temperature, ASFdoping):

TABLE 1 Catalyst T_(act.) [° C.] ASF doping³⁾ Hold time 1 600 1% byweight of ASF 2 h during act.² 2 600 2% by weight of ASF 2 h during act.3C¹⁾ 600 — 2 h 4 650 1% by weight of ASF 2 h during act. 5 650 2% byweight of ASF 2 h during act. 6C¹⁾ 650 — 2 h ¹⁾comparative experiment²⁾% by weight of ASF, fluoride content in ASF, based on the total massof the catalyst, which are added during activation ³⁾ASF = ammoniumhexafluorosilicatePolymerization Experiments

The polymerization experiments are carried out in a 0.2 m³ PF loopreactor (PF=particle-forming loop reactor). The melt flow rate (HLMFR: 8to 10 g/10 min) and the density (0.945 to 0.948 g/cm³) are set via thehexene concentration or ethene concentration in the suspension medium(isobutane). All catalyst variants are polymerized at a constant reactortemperature of 103.8° C. The reactor pressure is 39 bar. The polymeroutput is from 22 to 24 kg/h. For optimum mixing of the contents of thereactor, a Grassel pump is operated at 2100 rpm.

Table 2 shows the reactor temperature and the polymer analysis of thebatches prepared in the 0.2 m³ loop reactor using ASF-modified catalystsand for comparative experiments (C).

TABLE 2 P/C [g/g T_(act.) HLMFR [η] Density BD M_(w) ⁵⁾ M_(n) ⁶⁾Catalyst of cat.]¹⁾ [° C.] [g/10′]²⁾ [dl/g]³⁾ [g/ml] [g/l]⁴⁾ [g/mol][g/mol] Q⁷⁾ 1 3500 103.8 5.9 4.15 0.9437 527 400216 19964 20.0 2 4800103.8 5.4 3.96 0.9443 520 422888 25960 16.3   3C 2800 103.8 6.4 4.140.9448 522 400053 18359 21.7 4 4100 103.8 9.4 3.64 0.9443 509 32275325687 12.6 5 5100 103.8 8.3 3.56 0.9438 510 330264 21140 15.6   6C 3300103.8 8.1 3.81 0.9454 511 371850 17623 21.1 ¹⁾ratio of polymer/catalyst;²⁾melt flow rate; ³⁾viscosity; ⁴⁾bulk density; ⁵⁾weight averagemolecular weight; ⁶⁾number average molecular weight;

Table 3 shows the notched impact toughness (azk) and the environmentalstress cracking resistance (ESCR) of polyethylene prepared using thecatalysts of the invention and using comparative catalysts.

TABLE 3 azk²⁾, azk²⁾, 23° C. −30° C. Catalyst ESCR¹⁾/[h] [kJ/m²] [kJ/m²]1 43³⁾ 251 177 (>576⁴⁾) 2 46³⁾ 276 191 3C 33³⁾ 222 147 (>576⁴⁾) 5 24³⁾ —— 6C 19³⁾ 195 127 ¹⁾environmental stress cracking resistance; ²⁾notchedimpact toughness; ³⁾circular ESCR disks of ZKP/P at 80° C./3 bar;⁴⁾circular ESCR disks of ZKP/P at 50° C./3 bar

The polyethylene prepared using the catalysts of the invention has, inparticular, a high environmental stress cracking resistance combinedwith high notched impact toughness compared to the polyethylene preparedin the comparative experiments.

Test Methods

The melt flow rate (HLMFR=high load melt flow rate) was determined inaccordance with ISO 1133 at 190° C. under a load of 21.6 kg (190°C./21.6 kg).

The density [g/cm³] was determined in accordance with ISO 1183.

The Staudinger index (η) [dl/g] was determined in accordance with ISO1628 (at 130° C., 0.001 g/ml in decalin).

The bulk density (BD) [g/l] was determined in accordance with DIN 53468.

The impact toughness (azk) [kJ/m²] was determined in accordance with ISO180/1A.

The environmental stress cracking resistance (ESCR) was determined bythe round disk dart test (RD) of Elenac CAL in accordance with themethod laid down in the QM manual of Elenac CAL Test conditions: 50° C.or 80° C./3 bat/5% of Lutensol FSA, 10/2 mm pressed plate (scored).

The determination of the molar mass distributions and the means (M_(n),M_(w) and M_(w)/M_(n) derived therefrom was carried out by means ofhigh-temperature gel permeation chromatography (GPC) using a methodbased on DIN 55672 under the following conditions: solvent:1,2,4-tri-chlorobenzene, flow: 1 ml/min, temperature: 140° C.,calibration using PE standards.

1. A process for preparing a catalyst for the polymerization of olefins,which comprises: a) preparation of a finely divided silica xerogel bya1) use of a particulate silica hydrogel which contains from 10 to 25%by weight of solid (calculated as SiO₂), is largely spherical and has aparticle diameter of from 1 to 8 mm and is obtained by a11) introductionof a sodium or potassium water glass solution into a rotating stream ofan aqueous mineral acid, both longitudinally and tangentially to thestream, a12) spraying of droplets of the resulting silica hydrosol intoa gaseous medium, a13) allowing the sprayed hydrosol to solidify in thegaseous medium, a14) freeing the resulting largely spherical hydrogel ofsalts without prior aging by washing, a2) extraction of at least 60% ofthe water present in the hydrogel by means of an organic liquid, a3)drying of the resulting gel up to until the dried product experiences nofurther weight loss at 180° C and a reduced pressure of 13 mbar nofurther weight loss occurs over a period of for 30 minutes (xerogelformation) a4) adjustment of the particle diameter of the xerogelobtained to from 20 to 2000 μm, b) loading of the xerogel with chromiumfrom a solution of chromium trioxide or a chromium compound which isconverted into chromium trioxide under the conditions of step c), and c)activation of the resulting product at from 400 to 1100° C. in awater-free gas stream comprising oxygen in a concentration of above 10%by volume, wherein a fluoride doping with a fluorinating agent iscarried out in step c) together with the activation at 400 to 900° C. inair.
 2. The process as claimed in claim 1, wherein a silica hydrogelhaving a solids content of from 12 to 20% by weight is used in step a).3. The process as claimed in claim 2, wherein the extraction in step a)is carried out by means of an organic liquid selected from the groupconsisting of C₁-C₄-alcohols and C₃-C₅-ketones.
 4. The process asclaimed in claim 3, wherein the extraction in step a) is carried out bymeans of an organic liquid containing less than 3% by weight of water.5. The process as claimed in claim 4, wherein the application of thechromium trioxide to the xerogel in step b) is carried out from a0.05-15% strength by weight solution of chromium trioxide in aC₃-C₅-ketone or from a 0.05-15% strength by weight solution of achromium compound which is converted into chromium trioxide under theconditions of step c) in a C₁-C₄alcohol, where the respective solventmust contain no more than 20% by weight of water.
 6. The process asclaimed in claim 5, wherein the fluorinating agent is (NH₄)₂SiF₆,NH₄BF₄, (NH₄)₂AlF₆, (NH₄)₃PF₆ or NH₄HF₂.
 7. The process as claimed inclaim 6, wherein the fluorinating agent is used in an amount of from 0.5to 10% by weight, based on the total mass of the catalyst.
 8. A catalystfor the polymerization of olefins which is obtained by the process asclaimed in claim
 7. 9. The process as claimed in claim 1, wherein theextraction in step a) is carried out by means of an organic liquidselected from the group consisting of C₁-C₄-alcohols and C₃-C₅-ketones.10. The process as claimed in claim 1, wherein the extraction in step a)is carried out by means of an organic liquid containing less than 3% byweight of water.
 11. The process as claimed in claim 1 wherein theapplication of the chromium trioxide to the xerogel in step b) iscarried out from a 0.05-15% strength by weight solution of chromiumtrioxide in a C₃-C₅-ketone or from a 0.05-15% strength by weightsolution of a chromium compound which is converted into chromiumtrioxide under the conditions of step c) in a C₁-C₄-alcohol, where therespective solvent must contain no more than 20% by weight of water. 12.The process as claimed in claim 1, wherein the fluorinating agent is(NH₄)₂SiF₆, NH₄BF₄, (NH₄)₂AlF₆, (NH₄)₃PF₆ or NH₄HF₂.
 13. The process asclaimed in 1, wherein the fluorinating agent is used in an amount offrom 0.5 to 10% by weight, based on the total mass of the catalyst. 14.A catalyst for the polymerization of olefins which is obtained by theprocess as claimed in claim 1.